The present invention is for method and device for heating of billets and other heating of metallic materials for subsequent working.
Cast billets of steel and other metal alloys which shall be rolled or worked upon in other ways often have to be heated before these operations, this procedure is named billet heating. Also plates made from steel, aluminium and other metals have to be heated before rolling or other working. The temperature of the blank which one desires to achieve varies depending upon the composition of the alloy and other factors, for certain aluminium alloys from about 400xc2x0 C. and up to 1200-1300xc2x0 C. or more for alloys which are intended to be used at high operating temperatures. In order to create good conditions for the following procedure the temperature of the blank ought to be as uniform as possible.
It is known in the art to use heating furnaces where the source of heat is combustion or electrical resistance elements, for heating of billets so called walking beam furnaces are often used. In order to obtain a uniform distribution of the heat in electrically heated furnaces the elements are positioned at the walls and or ceiling of the furnace and often covers major parts of them. Also other kinds of electrically heated heat sources such as tungsten lamps have been used to a limited extent for some special purposes. In a conventional furnace, most often a walking beam or pusher type furnace, the blank rests on walking beams or a xe2x80x9ccoldxe2x80x9d bottom. This causes large variations of the temperature in the blank, especially during the initial heating phase. For this reason the blanks are often deformed and may sometimes look like bananas. These kinds of furnaces also in most cases have a long delay at changes of the temperature why resetting from one operating temperature to another will be time consuming.
A uniform and simultaneous heating of the blank may have deciding importance for the final result when it goes about metallurgically advanced alloys. HF-heating is sometimes used for blanks having homogenous cross section. The advantage thereof is the compactness of the heater, the disadvantage is also in this case the difficulty to achieve a uniform heating. The water cooling which is required takes a lot of energy and a poor power factor (cos xcfx86) will be the result unless large condensor batteries are used.
It is also known to use heaters where the heat source is IR-radiators having tungsten lamps and air-cooled reflectors. The use of these is limited to typical low temperature applications, up to 4-500xc2x0 C., e.g. preheating of aluminium blanks before extrusion. Already at these temperatures xe2x80x9ccounter radiationxe2x80x9d is a problem, the air cooling has to be increased to be sufficient for lamps and reflectors, and consequently the efficiency becomes low.
It is the object of the present invention to obtain a device for heating of billets and other heating of metallic materials be means of which the said disadvantages can be avoided or essentially reduced. It is thus one object of the invention to enable rapid and uniform heating of the billet or the material so the time for equalisation of the temperature after heating will be as short as possible. It is a further object of the invention to enable rapid temperature resettings and other adaptions to various blanks and alloys. It is a further obct to rapidly reach a balanced temperature. It is also an object of the invention to obtain energy saving relative to other kinds of heating device due to a good overall efficiency. In the following billets and blanks and heating of billets and blanks shall be understood to include also other metallic bodies and various situations of heating of metallic material before working.
The device according to the invention comprises modules in the shape of hoods a number of which as required, one or more, is placed over the blank which is to be heated. A module according to the invention comprises a hood made from fibrous material. Inside the hood there are built in one or more electrical heating elements so that due to reflection of IR-radiation from the insulation of the walls heating will take place symmetrically on all sides of the blank. The element modules and the walls are designed so that as uniform heat transfer to the blank as possible shall be brought about. Multiple element modules are used depending upon the length of the blank in the case of a batch furnace, or the necessary time inside the furnace in case of a continuous furnace. The modules or hoods are placed above a furnace bottom which is so designed that it will reflect heat radiation to the sides and bottom of the blank if it is placed on suitable supports or other means so that it does not rest directly on the bottom of the furnace. In order to achieve this the device is made so that major parts of the walls of the modules and the bottom of the furnace are at an angle to a vertical plane so that the reflected radiation is directed at the blank.
One advantage of the proposed design is the possibility of rapid temperature resettings and flexibility. This is of special importance in production where several alloys are processed which require different temperatures. It is also possible to achive a heat balance rapidly as an optimal low weight and efficient insulation has been selected. This also brings energy saving with it as the set working temperature is reached rapidly without preceding hold heating. The consequences of standstill due to exchange of elements and repair of wall covering will be small compared to using a large furnace of walking beam or push types. Several units of the proposed design are intended to replace a larger furnace of one of said kinds. For higher temperatures the best and economically most feasible solution is ceramic elements with reflectors made from ceramic fibres.
The method for heating and the heating device according to the invention and embodiments thereof have the characteristics which are mentioned in the claims.